The Old Man and the Sea Donald Geesaman Achievements and New Directions in Subatomic Physics 15 February 2010

2 In the quark model and QCD, it seems like the valence quarks and glue get all the respect  Valence quarks determine the charge and flavor of hadrons  Seem to explain the magnetic moments.  We thought, until 1990, that the valence quarks carried the spin  New accelerators, like the JLAB 12 GeV upgrade get built to study high x quarks  The glue dominates hadron structure at low x  New accelerators, like the electron-ion collider are planned to study the glue.

3 Maybe the sea quarks will go away! Motivated by desire to link to constituent quark or bag models, the hope was that as some low scale, Q, of a few hundred MeV/c, valence-like quark distributions plus glue would describe the nucleon, and the sea could be radiatively generated. Gluck, Godbole, and Reya (Z. Phys. C, 66 (1989) g u u It was then realized that some valence-like sea was needed. GRV, ZPC53, 127(92) Then it was found that the sea was not flavor symmetric.

4 Most of the information on the sea came from deep-inelastic lepton scattering, especially charged current neutrino experiments Q 2 = (k-k ’ ) 2 = mass 2 of the virtual boson x= Q 2 /(2m ) is the fractional momentum nucleon carried by the parton = E beam - E scattered y = / E beam muon and electron scattering~ charge current scattering ~ anti- c. c. scattering~ parity violating scattering, F 3 ~ parity violating anti- scattering~ The high statistics experiments are all done on nuclear targets

5 FNAL E866 Drell-Yan measurements on hydrogen and deuterium determined the x dependence of d-u Towell et al. Phys. Rev. D 64, (2001) Q 2 =54 GeV 2 Small but very important Since this is a flavor non-singlet quantity is true at all scales.

6 The simplest explanation is the pion cloud LA-LP The proton spends part of its time as a neutron plus π + |P> = α|uud> + β|udd> |uđ> We know pion cloud effects are important in quark models.

7 Of course Tony Thomas and his collaborators knew all this. Indeed they invented much of it.  1972 Sullivan  1980 Cloudy Bag Model Pions have to be included to preserve chiral symmetry in bag or bag-like models  1983Tony used the calculated pionic content and measured DIS to conclude that the fraction of the momentum of the nucleon carried by pions was 5+/-1.5% and was consistent with a bag radius of /-0.10 fm. Even today this is not such a bad representation of The problem is it also predicts the ratio as x goes to 1 from the charged and neutral pion Clebsch-Gordan coefficients

8  pQCD - Gluon splitting?  Meson Cloud? Chiral Solitons? Instantons?  Models describe well, but not — pQCD becoming dominant???? LA-LP Structure of the nucleon: What produces the nucleon sea? Peng et al. No one has come up with a physical mechanism to make

9 A key seems to be the spin carried by the non- singlet anti-quarks E866 Pion content – flavor non-singlet anti-quarks carry 0 net spin. Pions do affect the spin carried by the quarks through their interaction with the remnant baryon Statistical Model - Bourelly and Soffer Instanton Chiral quark-Soliton - Dresslar et al. EPJC18, 719 (2001) gives similar result.

10 What are the correlations between the q and q pairs in the sea?  Gluon 1 -, 3 S 1 Flavor neutral  Meson 0 -, 1 +  Vacuum 0 +, 3 P 0 Flavor neutral g

11 What do the data tell us ?  E866 - PR D64, (2001) Q 2 =54 GeV 2  HERMES - PR D71, (2005)  COMPASS- arXiv: v1 Q 2 =3 GeV 2  de Florian et al - PRL 101, (2008) Q 2 =10 GeV 2 3 σ from zero 2 σ from.197=Chiral soliton To be compared with 0, -1, -5/3 * flavor asymmetry

12 COMPASS and HERMES Data DNS 2005 DSSV to -.19

13 What does Tony say now? Myhrer-Thomas picture of proton spin  Relativistic valence quarks - orbital motion accounts for 35%  quark-quark hyperfine interaction  Pion cloud Only the hyper-fine interaction could contribute to so I believe the prediction is small.

14 HERMES has a new slant on the strange quark distributions. A. Airapetian et al Phys. Lett. B 666, 446 (2008) Usually s(x)+sbar(x) ~ κ (ubar+ dbar) with κ~ 0.5 Best handle has been considered to be multi-muon events in neutrino scattering. HERMES looks at polarized DIS on deuterium and compares inclusive with semi- inclusive kaon multiplicities

15 HERMES sees little strange quark content for x>0.1 and s(x)+sbar(x) ~ ubar(x)+dbar(x) at x< 0.03! A. Airapetian et al Phys. Lett. B 666, 446 (2008) Q 2 =2.5 GeV 2

16 How is this consistent with years of neutrino multi-muon data? ν + s → μ + + c →μ - NUTEV, PRD (2001) CTEQ, JHEP 42, 89 (2007)

17 NuTeV Data Suggest Small Strange vs Anti-strange Asymmetry PRL 99, (07)

18 Comparison of ubar+dbar-s-sbar with dbar-ubar Based on the HERMES result and assuming the strange quark distribution represents the gluon-splitting induced distribution, the shape of the non-perturbative is similar to vs 0.25 *HERMES

19 Nuclear corrections in charged lepton and neutrino scattering are different Charged lepton Fe/DNeutrino Fe/D

20 Parton Distributions in Nuclei  1984 – Parton distributions are different EMC effect – nucleon carries smaller fraction of momentum or changes structure Shadowing  Expected large pion-cloud effects  1990 – little change in sea quarks for x>0,1 Alde et al (Fermilab E772) Phys. Rev. Lett (1990) My one publication with Tony 6 th on his citation list

21 Our visual images of a nucleus OR “nucleons” held apart by short range repulsion but even in 208 Pb, half the nucleons are in the surface average spacing at ρ nm ~ 1.8 fm Radius of a nucleon ~ 0.8 fm average spacing at 3ρnm ~ 1.3 fm Remember 1983 Thomas result favored a bag radius of 0.9 fm

22 We want to describe a nucleus  Hadronic Description –exemplified by ab initio calculations with potentials NN NNN + NNNN + Bare form factors Meson exchange currents  Past two decades have shown this is remarkably successful  Pure QCD Description –what are the clusters of quarks in a nucleus? –know the parton distributions change EMC effect shadowing x>1  One problem is always whether our description of a bare proton is good enough. The second is how to actually calculate many body effects beyond mean field?

23 x target x beam Use a proton beam: primarily u quarks at high x. Detector acceptance chooses x target and x beam.  Fixed target  high x F = x beam – x target  Valence Beam quarks at high-x. (e 2 u)/(e 2 d) > 8 Dominated by u quarks  Sea Target quarks at low/intermediate-x. Drell-Yan scattering: A laboratory for sea quarks E906 Spect. Monte Carlo

24 Advantages of 120 GeV Main Injector The (very successful) past: Fermilab E866/NuSea  Data in  1 H, 2 H, and nuclear targets  800 GeV proton beam The future: Fermilab E906  Data taking  1 H, 2 H, and nuclear targets  120 GeV proton Beam  Cross section scales as 1/s –7 x that of 800 GeV beam  Backgrounds, primarily from J/  decays scale as s –7 x Luminosity for same detector rate as 800 GeV beam 50 statistics!! 50 x statistics!! Fixed Target Beam lines Tevatron 800 GeV Main Injector 120 GeV

25 Fermilab E906/SeaQuest Collaboration Abilene Christian University Donald Isenhower, Mike Sadler, Rusty Towell, Shon Watson Academia Sinica Wen-Chen Chang, Yen-Chu Chen, Da-Shung Su Argonne National Laboratory John Arrington, Don Geesaman *, Kawtar Hafidi, Roy Holt, Harold Jackson, David Potterveld, Paul E. Reimer *, Josh Rubin, Patricia Solvignon University of Colorado Ed Kinney Fermi National Accelerator Laboratory Chuck Brown, Dave Christian University of Illinois Naomi C.R Makins, Jen-Chieh Peng KEK Shin'ya Sawada Kyoto University KenIchi Imai, Tomo Nagae * Co-Spokespersons Ling-Tung University Ting-Hua Chang Los Alamos National Laboratory Gerry Garvey, Xiaodong Jaing, Mike Leitch, Ming Liu, Pat McGaughey, Joel Moss University of Maryland Prabin Adhikari, Betsy Beise, Kazutaka Nakahara University of Michigan Wolfgang Lorenzon, Richard Raymond RIKEN Yuji Goto, Atsushi Taketani, Yoshinori Fukao, Manabu Togawa Rutgers University Lamiaa El Fassi, Ron Gilman, Elena Kuchina, Ron Ransome, Elaine Schulte Texas A & M University Carl Gagliardi, Robert Tribble Thomas Jefferson National Accelerator Facility Dave Gaskell Tokyo Institute of Technology Toshi-Aki Shibata, Yoshiyuki Miyachi

26 Projected errors on ratios of D to H Errors on ratio ~ 1% until statistics become a factor. The absolute cross section on deuterium measures Errors limited by beam normalization and acceptance ~ 5%

27 Does deuterium structure affect the results at higher x

28 Structure of nucleonic matter:  Nucleon motion in the nucleus tends to reduce parton distributions – f(y) peaked below y=1.  Rescaling effects also reduce parton distribution for x>0.15  Antiquark enhancement expected from Nuclear Pions.  This data also constrains the maximum effects for deuterium.

29 Summary  The origin and structure of the sea remain critical themes in the physics of the nucleon and nucleus  We need to push to higher x values and E906/SeaQuest is especially well suited for this. We start this summer and run for two years.  The other really key measurement is improved precision in the spin carried by the sea quarks and the spin-correlations in the sea. –COMPASS, RHIC, J-PARC, JLAB 12 GeV  This is difficult and may require the next generation of polarized Drell-Yan experiments  Whatever we measure, Tony Thomas will have thought of it first and helped stimulate the experiments  And there is a chance, he may even have got it right.

30 The models all have close relations between antiquark flavor asymmetry and spin Statistical Parton Distributions